The present invention relates generally to the recovery of heat from hot gas streams containing entrained hot fine particles. More particularly, the present invention relates to a circulating bed heat exchanger for recovering heat from the hot flue gas produced during dense phase fluidized bed combustion of spent shale.
Oil shale is a marlstone-type mineral having varying amounts of an organic complex known as kerogen dispersed therein. Vast reserves of oil shale are known to be present in major portions of Utah, Wyoming and Colorado. The extent of these reserves has been estimated at between three and seven trillion barrels of shale oil. The presence of these vast reserves of oil shale has sparked intense interest in developing processes for recovering useful oil and gas from this inorganic mineral-organic complex mixture.
As is well known, in order to convert the organic polymer kerogen into a commercially useful form, the kerogen must be decomposed and separated from the inorganic components of oil shale. The majority of present processes which appear commercially feasible involve heating the oil shale in a reducing atmosphere to pyrolyze the kerogen to form volatile oils and gaseous hydrocarbon products.
Typically, the raw oil shale is dried and preheated, if desired, to temperatures between 200.degree. F. and 600.degree. F. The preheated oil shale is then passed to a retort where it is heated to temperatures in the range of 800.degree. F. to 1100.degree. F. At these temperatures, the kerogen thermally decomposes to form volatile hydrocarbon products. These volatile products are separated from the remaining inorganic oil shale residue and recovered in suitable condensors. The residue remaining after pyrolysis is commonly referred to as spent shale and typically includes up to about 5 percent to 10 percent by weight combustible carbonaceous residue and may contain even higher amounts depending upon the particular oil shale feed and retorting conditions.
An important aspect of any pyrolysis process is the means by which the raw oil shale is heated to pyrolysis temperatures. Numerous different heating systems have been devised including indirect heating, direct heating with hot fluid gases and direct heating by mixture with hot heat-carrying bodies. The use of heat-carrying bodies for direct heat transfer to the oil shale has been found to be an especially effective way to heat the oil shale solids. The types of heat carriers developed for use in pyrolysis systems range widely in composition, size and structure. Heat carriers ranging in size from relatively large spherical ceramic balls down to relatively small particles of sand, minute alumina beads, attrition-resistant shale ash and other particulate solids are among the materials which have been found suitable for use as heat carriers.
As is apparent, after heat transfer and the resultant partial cooling in the retort, the heat carrier must be reheated prior to recycling. Numerous different heat carrier circuits have been developed for providing heating, separation and classification of the heat carrier particles. One especially convenient source of heat for reheating the heat carrier solids is provided by combusting the carbonaceous residue present in spent shale. Combustion is generally carried out at temperatures in the range of 1100.degree. F. to 1700.degree. F. and produces hot shale ash particles which contain little if any combustible carbon residue.
During spent shale combustion, the spent shale and shale ash particles undergo attrition to varying degrees depending upon combustion conditions and the attrition resistance of the shale. Attrition resistance of various spent shales and shale ashes depends, to a large extent, upon the kerogen content of the initial oil shale feed. Rich oil shales tend to produce pulverulent spent shale and shale ash which break down readily to produce fine particles. When the spent shale residue from retorting rich shales, such as those from the Green River formation are combusted in a fluidized bed combustion unit, essentially all of the ash from the combustor is entrained in the combustor flue gas. On the other hand, kerogen-lean shales produce a relatively attrition resistant spent shale and shale ash which does not readily decrepitate. Even for the very lean oil shales, however, a certain amount of attrition will occur during retorting and combustion to produce fine particles of shale ash which are entrained in the spent shale combustor flue gas.
The fine particles of shale ash at the elevated temperatures of spent shale combustion are considered a waste product and are typically discarded from the oil shale pryolysis system. In order to insure maximum energy efficiency for the process, it is important that the heat content of the shale ash fines and combustor flue gas be recovered prior to disposal by cooling the flue gas and ash fines.
A common way to obtain this cooling is to separate the ash fines from the flue gas and to cool each stream separately. The flue gas is usually cooled in a tubular exchanger while the ash is cooled in a rotary tube cooler, fluid bed or other solids cooler.
The problem with this type of two stream cooling process is that the ash particle size range is very broad and much of the ash is fine (less than 150 mesh). Due to this small size, it is difficult to remove all ash from the flue gas. Cooling the separated ash is also a problem due to the ease with which the ash can become airborne, the high potential for the fine ash to foul heat transfer surfaces and the low heat transfer coefficients which usually result from such fouling. Also, the separated ash is difficult to fluidize and is difficult to convey.
It has been found that the problems associated with the handling and cooling of spent shale combustor flue gas are reduced if the ash and flue gas are cooled simultaneously. This simultaneous flue gas and entrained ash cooling may be conducted in tubular heat exchangers of the type used for cooling the flue gases from steam boilers in utility plants. Although simultaneous cooling of ash and flue gas is desirable, it has been found that the heat transfer coefficient during simultaneous flue gas and ash cooling is much less than that achievable during separate cooling of the separated ash. For example, the heat transfer coefficient from a dense phase fluidized bed of shale ash to a steel tube immersed in the bed is usually on the order of 30 to 50 Btu/ft.sup.2 .degree.F. per hour. The heat transfer coefficient for the same tube, temperature and gas flow, but with the reduced level of solids typically present in combustor flue gas, is greatly reduced. In addition, the fine shale ash particles tend to coat the heat exchanger walls to form an insulating layer of ash which further reduced heat transfer and cooler efficiency.
It would therefore be desirable to provide a simultaneous flue gas and entrained ash cooling system and process in which the heat transfer coefficient from the flue gas and entrained fine solids is maximized. It would also be desirable to provide such a process in which the formation of an insulating coating of shale ash fines on the heat exchanger surfaces is reduced.